A typical conventional three-phase motor driving apparatus detects magnetic poles of three phases of a rotor with three rotor sensors, and acquires information on the rotational position of the rotor based on three pulse signals output by the rotor sensors based on the detection result.
The conventional three-phase motor driving apparatus, which uses three expensive rotor sensors, requires harnesses and terminals to be connected to the rotor sensors and therefore has a problem that the manufacturing cost of the system increases.
In addition, the rotor sensors have to be positioned with high precision in order to output precise detection signals. Therefore, there is also a problem that the yield of the products decreases as the number of rotor sensors increases.
For example, the three-phase motor driving apparatus described in JP2010-239748A estimates the rotational position of the rotor with one rotor sensor to drive a three-phase brushless motor.
Specifically, with the three-phase motor driving apparatus, in a reference energization cycle in which the U phase, the V phase and the W phase are configured to be shifted from each other by an electrical angle of 120°, a half of the cycle following the time when the U phase as a reference is turned on or off is equally divided into three phases, a former phase, a middle phase and a later phase. Based on these phases, the state of energization of the three-phase brushless motor is controlled.
With the three-phase motor driving apparatus described in JP2010-239748A described above, a half cycle is always equally divided into three phases, the former phase, the middle phase and the later phase, and therefore the influence of a variation of the rotation speed of the three-phase brushless motor is not taken into account.
That is, the three-phase motor driving apparatus cannot properly estimate the rotational position of the rotor if the rotation speed of the three-phase brushless motor varies.